Graft versus host disease (GVHD) is a complex pathophysiological process that has been divided into acute and chronic phases that are both temporally and clinically distinct. Since in most patients, chronic GVHD develops in the setting of preexisting acute GVHD, a longstanding unresolved issue has been how acute GVHD which is initiated by alloreactive donor T cells evolves into chronic GVHD where autoreactive donor T cells have been speculated to play a role in the pathophysiology of this syndrome. In preliminary studies, we have shown that, during the course of GVHD, there is breaking of tolerance to self antigens, that this occurs in the setting of ongoing alloreactivity, and that this process is critically dependent upon antigen presentation by donor APCs. These preliminary data lead us to hypothesize that autoreactivity is an integral component of GVHD, evolves from a strong antecedent alloresponse, and contributes to pathological damage in transplant recipients. However, there remain many unresolved questions. These include the relationship between T cells involved in autoreactivity versus alloreactivity, the temporal kinetics of autoreactivity in transplant recipients, the role that impaired or absent T cell regulation plays in the pathophysiology of autoreactivity, and the role of donor APCs in propagating both autoreactive and alloreactive responses. To address these issues, experiments have been designed to address the following specific aims: (1) to determine whether distinct donor T cell populations are responsible for mediating alloreactivity and autoreactivity, (2) to define the temporal kinetics whereby donor T cells from GVHD mice acquire the ability to respond to self antigens, (3) to determine whether autoimmunity is attributable to an absent or impaired regulatory response, and (4) to define the role of donor APCs in the induction of autoimmunity and alloimmunity in secondary recipients. The overall goal of this proposal is to provide new insights into the pathophysiology of GVHD and to resolve the paradox for how alloreactivity evolves into autoreactivity in GVHD recipients. The relevance of this project to public health derives from the fact that GVHD is the major complication of stem cell transplantation. Greater understanding of this complex process which we hope will come from these proposed studies will result in better therapies which will improve overall survival and expand the number of patients that might benefit from stem cell transplants.